The nature raid is really a big hurdle, especially girl with oneshotting curses and sucking entire mana from party. The fairy girl after some number of defeats will propose you a challenge battle
completing which will give you a choice between few rewards. That's where you can choose to get key or seggs or other options.
So about advices: I think the comfortable level for all raids is 50 and you should farm fire and ice raids before stepping in nature one. Max out your main party members story events to promote them and gain new skills.
In the end might as well stack on some consumables like stimulant/elixir/iced coffee(don't rely on Mira's reviving too much, consumables are more effective) because curse girl is merciless(or rather all nature raid girls
).